(1) Field of Invention
Method of pleating sheet material.
(2) Prior Art
In my U.S. Pat. No. 3,349,159, issued Oct. 24, 1967, a method and apparatus are disclosed for pleating transparent plastic sheet material of the thickness disclosed therein and in my U.S. Pat. No. 3,257,486, issued Jan. 21, 1966, referred to in U.S. Pat. No. 3,349,159. Therein the sheet material is pulled from a roll lengthwise, passed by short successive advances through a pleating device by which, between advances, it is pleated, and from which it is passed immediately between heated compression rolls which heat and iron the pleats into flat condition.
It had been found that, if, before setting, the sheet were unconfined flatwise, it could not be pulled along its path adequately by endwise tension as the tensioning force opened and pulled out the pleats so that they would become set in partially open or overextended condition. To maintain the pleated sheet in the flat pressed condition from the instant of pleating, during heating, and until the pleats are cooled and set, the pleated sheet is confined between ironing or confining sheets of paper which are drawn from suitable rolls and pass between the heating and pressing rollers while in fixed direct face to face contact with opposite faces, respectively, of the pleated sheet. These confining sheets remain in contact with the pleated sheet, one at each face, until the pleats are set, and then, they are separated from the plastic sheet and rewound into rolls for reuse.
With very thin plastic sheets, such as from 1 to 10 mils in thickness, as disclosed in said U.S. Pat. Nos. 3,349,159 and 3,257,486, consistency in the finished product is difficult to obtain at all times by this prior method and machine. The thin sheets sometimes gather, wrinkle, or pull and stretch to different degrees at different localized areas with resultant unwanted variations in shape and inaccuracy in dimension. The sometimes erratic behavior of the thin sheets seemed to result not only from the low resistance of the sheet to wrinkling and to localized stretching, but also from slight differences in moisture in the sheet at different times and areas, and from differences in the humidity of the atmosphere at the immediate site. Furthermore, since the pleating is done one sheet at a time, the production is limited.